1. Field of the Invention
The present invention relates to a communication system using an orthogonal frequency division multiplexing (OFDM) scheme, and more particularly to an apparatus and a method for performing dithering in a OFDM-based communication system.
2. Description of the Related Art
Generally, the procedure for converting an analog signal into a digital signal in a communication system is divided into sampling, quantizing, and coding.
The sampling refers to a procedure of obtaining samples in order to convert continuously changing analog signals into a digital signal having a discrete form. A sampling rate refers to the number of samples per second (the number of digitization). The unit of the sampling rate is hertz “Hz” (frequency unit).
The quantizing refers to a procedure of approximating to the nearest integer value each signal point sampled according to the sampling rate. In other words, the sampling is a digitization procedure over time, and a digitization procedure for signal intensity is quantizing. When the analog signal is expressed as quantization bits through the sampling, errors occur. During this procedure, the difference that may occur between an input signal and a quantized output signal is referred to as a “quantization error”. The number of quantization bits must increase in order to minimize the quantization error. However, if the number of quantization bits increases, system or terminal realization costs increase. In addition, as the number of quantization bits increases, the number of computations for the quantization also increases.
Dithering is a scheme in which a wide-range signal can be digitized or low-bit quantized without signal distortion by increasing an available dynamic range in the process of converting an analog signal into a digital signal even when the signal is digitized or quantized by using a smaller number of quantization bits.
In more detail, the dithering is a scheme in which quantization noises and sound distortion can be minimized by artificially adding white gaussian noise or dithering noise in the process of converting an analog signal into a digital signal. Accordingly, the low-bit quantization can be achieved by adding noise to a high-bit quantized signal when converting an analog signal into a digital signal.
Hereinafter, a phenomenon in which harmonics appear when dithering noise is not added will be described using graphs with reference to FIGS. 1A and 1B.
FIGS. 1A and 1B show a typical signal waveform when dithering noise is not added.
FIG. 1A shows a graph illustrating a waveform of a signal obtained by sampling a 1 kHz analog signal using a 44 kHz sampling frequency and then 8-bit quantizing the sampled signal. In the graph shown in FIG. 1A, an x-axis represents time, and a y-axis represents voltage intensity or current intensity.
FIG. 1B shows a graph illustrating a spectrum of a signal obtained by sampling a 1 kHz analog signal using a 44 kHz sampling frequency and then 8-bit quantizing the sampled signal.
Generally, an average voice falls within the audible frequency range of 15 kHz to 20 kHz, and a signal component having a frequency of above 20 kHz is included in an ultrasonic wave range which cannot be detected by human ears. However, it can be understood from FIG. 1B that a harmonic component interfering with a signal may have a power level much higher than an average quantization noise power level of −10 dB.
FIG. 2 shows a block diagram illustrating the structure of a typical receiver employing dithering.
The receiver receives an analog signal transmitted from a transmitter and adds to the received signal noise generated from a dithering noise generator 202. The dithering noise generator 202 includes a noise shaping filter 204 and a random noise generator 206. The random noise generator 206 generates random noise and outputs the random noise to the noise shaping filter 204. The noise shaping filter 204 removes interference of an audible frequency band using the dithering noise. The random noise having passed through the shaping filter 204 has only a non-audible frequency band component, so that interference does not occur in the audible frequency band.
A low-bit analog-digital converter (ADC) 208 receives a signal obtained by adding the dithering noise to the analog signal and converts the signal into a digital signal. It is assumed that the ADC 208 outputs the digital signal by performing low-bit quantization.
FIG. 3 is a graph illustrating a typical waveform of a signal with dithering noises.
As shown through the graph of FIG. 3, the signal obtained by adding the dithering noise to the analog signal has no harmonic component after an analog-digital converting procedure including low-bit quantizing.
An average power in a time domain is proportional to the number of sub-carriers in a typical OFDM system. For example, if the OFDM system employs a smaller number of sub-carriers, a received OFDM signal intensity in a time domain could be less than a minimum quantization level. In this case, OFDM system performance may be degraded. A description of the performance degradation will be given with reference to FIGS. 4A to 4C.
FIGS. 4A to 4C show conventional problems occurring when input signal intensity is lower than low-bit quantization level in a time domain.
FIG. 4A is a graph illustrating a waveform of a low-bit quantized signal when input signal intensity is less than a low-bit quantization level in a time domain.
FIG. 4B is a graph illustrating power spectral density (PSD) of an input signal when input signal intensity is less than a low-bit quantization level in a time domain.
FIG. 4C is a graph illustrating signal constellation after Fast Fourier Transform (FFT) when input signal intensity is less than a low-bit quantization level in a time domain.
As shown in FIG. 4C, when input signal intensity is less than a low-bit quantization level in a time domain, a Signal to Noise Ratio (SNR) of the input signal becomes 5.2 dB. When the input signal intensity is greater than the low-bit quantization level (in a case of a 8-bit quantization level) in the time domain, an ideal SNR is 20.8 dB, so the SNR of 5.2 dB represents the degradation of performance.
In addition, frequency components of dithering noise added to the received analog signal may be similar to a frequency component of the received analog signal. In other words, frequency components of the dithering noise may be close to sub-carrier indices of the received analog signal. In this case, the dithering noise acts as interference signals with respect to the received analog signal.